The present invention relates to microelectromechanical system (MEMS) actuators and, in particular, to thermal microelectromechanical system actuators that are activated by Joule heating.
Microelectromechanical system (MEMS) actuators provide control of very small components that are formed on semiconductor substrates by conventional semiconductor (e.g., CMOS) fabrication processes. MEMS systems and actuators are sometimes referred to as micromachined systems-on-a-chip.
One of the conventional MEMS actuators is the electrostatic actuator or comb drive. Commonly, such actuators include two comb structures that each have multiple comb fingers aligned in a plane parallel to a substrate. The fingers of the two comb structures are interdigitated with each other. Potential differences applied to the comb structures establish electrostatic interaction between them, thereby moving the comb structures toward and away from each other.
Advantages of the electrostatic actuator are that they require low current, which results in small actuation energy, and have a relatively high frequency response. Disadvantages are that they require high drive voltages (e.g., tens or hundreds of volts) and large areas and provide low output forces. Comb drive (electrostatic) actuators used for deployment of microstructures typically occupy many times the area of the device they are deploying. Also, the high voltages (e.g., tens or hundreds of volts) required to operate electrostatic actuators can be incompatible and prevent integration with conventional logic and low voltage electronics.
A pseudo-bimorph thermal actuator is an alternative to the electrostatic actuator. These actuators utilize differential thermal expansion of two different-sized polysilicon arms to produce a pseudo-bimorph that deflects in an arc parallel to the substrate. Such a thermal actuator produces much higher forces (100-400 times) per unit volume than comb drive actuators and can operate on very low voltages. Such actuators are limited to sweeping or arc motion in the plane of the actuator.
The present invention includes an out-of-plane thermal buckle-beam microelectrical mechanical actuator formed on a planar substrate of semiconductor material (e.g., silicon). The actuator includes first and second anchors secured to the substrate and multiple elongated thermal buckle beams that are secured between the anchors. The buckle beams are formed of semiconductor material, such as polysilicon. In one implementation, the buckling beams are coupled together by a pivot frame that includes a frame base secured to each buckle beam and at least one pivot arm that is coupled to the frame base at one end and includes a free end that pivots out-of-plane when the actuator is activated.
A cyclic current source directs cyclic electrical current through the thermal buckle beams via the anchors to impart thermal expansion of the buckle beams and hence a cyclic buckling motion of them out of the plane (i.e., away from) the substrate. In one implementation, the actuator has a characteristic resonant deflection frequency range and the cyclic current is of a first frequency within the resonant deflection frequency range.
Actuators according to the present invention provide out-of-plane motions with forces comparable to conventional thermal actuators. The resistivity of silicon allows the actuator to operate at voltages and currents compatible with standard integrated circuitry (e.g., CMOS). In addition, actuators according to the present invention are very small in area, have relatively high force. This electrically stimulated movement can be used in micro-motors, optical scanning devices, MEMS optical deployment mechanisms and other areas requiring mechanical movement on a micro scale. For example, an actuator structure of the present invention includes a pair or transverse actuators and an out-of plane fold mirror that cooperate to form a video raster scanner.
Additional objects and advantages of the present invention will be apparent from the detailed description of the preferred embodiment thereof, which proceeds with reference to the accompanying drawings.